This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The phytochrome (phy) family of sensory photoreceptors control plant adaptational responses to light signals from the environment. Current data indicate that the signal transduction process involves rapid translocation of the light-activated photoreceptor molecule from the cytoplasm to the nucleus, where it interacts physically with a subset of members of the bHLH transcription-factor family, termed phytochrome-interacting factors (PIFs), inducing transcriptional responses in target genes. Recent evidence shows that members of the PIF family repress development in the dark, and that photoactivated phy reverses this repression by inducing rapid degradation of the PIF molecules upon exposure to light. This process involves rapid, phy-induced phosphorylation of the interacting bHLH protein, followed by degradation via the ubiquitin proteasome system. The specific, primary focus of this segment of the project is to identify the light-induced phosphorylation sites in the PIF3 molecule, as a step toward understanding the mechanism of phosphorylation and the role it plays in tagging the bHLH factor for ubiquitylation. A second related objective is to identify proteins that are induced by light to interact with the PIF3 protein as a step toward identifying the protein kinase and E3 ligase responsible for the phosphorylation and ubiquitylation steps in the signaling process. It is proposed to use mass spec analysis to address both these objectives.